Hot plate unit

ABSTRACT

A hot plate unit for heating semiconductor wafers. The hot plate unit includes a case, a hot plate, a seal element, and a holding ring. The seal element is made of a heat insulative material and is arranged between the case and the hot plate. The hot plate is clamped between the holding ring and the seal element. Screws for fastening the holding ring do not contact the hot plate.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of (and incorporates by reference)patent application Ser. No. 09/471,759 filed Dec. 23, 1999 and entitled“HOT PLATE UNIT.”

BACKGROUND OF THE INVENTION

The present invention relates to a hot plate unit, and moreparticularly, to a hot plate unit for heating a silicon wafer during asemiconductor manufacturing process.

In a semiconductor manufacturing process, a hot plate unit is normallyused to dry photosensitive resin, which is applied to silicon wafers.The hot plate unit is incorporated in a semiconductor manufacturingapparatus.

Japanese Examined Patent Publication 4-13873 describes a first exampleof a prior art hot plate unit. The hot plate unit includes a hot plate,which is made of an aluminum nitride sintered body, and a resistorarranged in the plate. The two ends of the resistor project from thesides of the plate and are connected to a power supply via wires.

A silicon wafer, which is the heating subject, is placed on the hotplate. When the resistor is supplied with current, the resistor isheated for a predetermined time period to heat the silicon wafer and drythe photosensitive resin. The flow of current is then stopped togradually cool the hot plate and the silicon wafer. After the hot plateand the silicon wafer are cooled to a relatively low temperature, thesilicon wafer is removed from the hot plate to undergo the next process.Since the time period for cooling the hot plate is relatively long, itis difficult to improve productivity.

To decrease the cooling time of the hot plate, the plate may be cooledby, for example, forcing cooling air against the lower surface of thehot plate. However, the moisture and dust in the forced air maycontaminate the semiconductor manufacturing apparatus and itsenvironment.

Japanese Examined Patent Publication No. 8-8246 describes a secondexample of a prior art hot plate. The hot plate includes a cooling bodyprovided with cooling fins. However, although the cooling body cools thehot plate locally, the hot plate cannot be cooled entirely.

FIG. 1 shows a third example of a prior art hot plate unit 41. The hotplate unit 41 includes a cup-like casing 42 and a hot plate 43 made of aceramic sintered body. The casing 42 is made of a metal material, suchas aluminum, and has a lip 44 located at its upper portion. The casing42 has an opening encompassed by the lip 44. Resistors 45 are arrangedin a predetermined pattern at the lower surface of the hot plate 43. Aplurality of screw holes 46 extend through the peripheral portion of thehot plate 43. A plurality of threaded holes 47 extend through the lip44. The screw holes 46 of the hot plate 43 are aligned with the threadedholes 47 of the lip 44. A screw 48 is inserted into each set of alignedholes 46, 47 to fasten the hot plate 43 to the casing 42.

The hot plate unit 41 uses fasteners, such as the screws 48, to fastenthe hot plate 43 directly to the metal casing 42. Thus, the heat of thehot plate 43 is transmitted to the casing 42 through the screws 48. As aresult, the temperature at the periphery of the hot plate 43 tends to belower. Furthermore, the hot plate 43 cannot be cooled in a uniformmanner.

In addition, the fastening force of the screws 48 may damage the hotplate 43 since the force is applied directly to the hot plate 43.Further, it is difficult to machine threaded holes in the hot plate 43,which is made of a hard material.

SUMMARY OF THE INVENTION

Accordingly, it is a first object of the present invention to provide ahot plate unit that adjusts the temperature of a heating subject in auniform manner. It is a second object of the present invention toprovide a hot plate unit that is easily secured to a supporting body.

To achieve the above objects, the present invention provides a hot plateunit including a case and a hot plate having a heat generator. The caseand the hot plate define a space in which a fluid flows.

Another aspect of the present invention provides a hot plate unitincluding a case having a lip, a hot plate having a heat generator, anda holder for securing the hot plate to the case. The hot plate is madeof sintered ceramic. The holder contacts a peripheral portion of theupper surface of the hot plate when the holder is fixed to the case.

Further aspect of the present invention provides a hot plate unitincluding a case having a lip, a hot plate having a heat generator, anda seal element. The hot plate is made of sintered ceramic. The hot plateis secured to the case via the seal element. The seal element is heatinsulative.

Further aspect of the present invention provides a hot plate unitincluding a case having a lip, a hot plate having a heat generator, andat least one intake port extending through the case.

Other aspects and advantages of the present invention will becomeapparent from the following description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention that are believed to be novel areset forth with particularity in the appended claims. The invention,together with objects and advantages thereof, may best be understood byreference to the following description of the presently preferredembodiments together with the accompanying drawings in which:

FIG. 1 is a cross-sectional view partially showing a prior art hot plateunit;

FIG. 2 is an exploded perspective view showing a hot plate unitaccording to a first embodiment of the present invention;

FIG. 3 is a cross-sectional view showing the hot plate unit of FIG. 2;

FIG. 4 is an enlarged cross-sectional view showing the periphery of thehot plate unit of FIG. 3;

FIG. 5 is a cross-sectional view showing a hot plate unit according to asecond embodiment of the present invention;

FIG. 6 is a cross-sectional view showing a hot plate unit according to athird embodiment of the present invention;

FIG. 7(a) is a plan view showing a resistor pattern formed in a hotplate unit according to a fourth embodiment of the present invention;

FIG. 7(b) is a plan view showing the locations of a intake port and adischarge port in the hot plate unit of FIG. 7(a);

FIG. 8 is an exploded perspective view showing a hot plate unitaccording to a sixth embodiment of the present invention; and

FIG. 9 is a cross-sectional view showing a hot plate unit according toanother example of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A hot plate unit 1 according to a first embodiment of the presentinvention will now be described with reference to FIGS. 2 to 4.

As shown in FIGS. 2 to 4, the hot plate unit 1 includes a hot plate 3and a metal (e.g., aluminum) case 2. The case 2 has a bottom wall 2 aand a lip 4. Three guide sleeves 5 (only two shown) are arranged at thecentral portion of the bottom wall 2 a. A lift pin (not shown) isinserted into each of the three lift pin holes 11. The three lift pinssupport a silicon wafer W1 at three points and lift the silicon wafer W1to a predetermined height from the upper surface of the hot plate 3. Alead wire hole 7 extends through a peripheral portion of the bottom wall2 a. Lead wires 6 for supplying current to the hot plate 3 extend fromthe case 2 through the lead wire hole 7.

The hot plate 3 includes a base plate 9 made of a ceramic sintered bodyand a heat generator 10 formed on the base plate 9. In this embodiment,the heat generator 10 is a resistor that is formed in a predeterminedpattern. The pattern is arranged for uniformly heating the entire baseplate 9. The pattern is preferably one of concentric circles or aspiral. A silicon wafer W1, to which a photosensitive resin has beenapplied, is placed on the hot plate 3 and heated to a predeterminedtemperature (e.g., 200° C. to 300° C.) to dry the photosensitive resin.As shown in FIG. 2, the case 2 and the base plate 9 are both circular.The diameter of the base plate 9 is slightly smaller than that of thecase 2. The base plate 9 is secured to the top of the lip 4 with anattachment or a seal element 14. Three lift pin holes 11 extend throughthe central portion of the hot plate 3. With reference to FIG. 4, thethickness 1 of the base plate 9 is preferably three millimeters but maybe changed to an arbitrary thickness within a range of one to tenmillimeters.

A nitride ceramic sintered body having superior heat resistance and highheat conductivity may be used as the material of the base plate 9. Apreferred nitride ceramic body is a ceramic sintered body made of ametal nitride, such as aluminum nitride, silicon nitride, boron nitride,or titanium nitride. A ceramic sintered body made of a metal carbide,such as silicon carbide, zirconium carbide, titanium carbide, tantalumcarbide, or tungsten carbide, may also be used. Among these materials,aluminum nitride has the highest heat conductivity and is thusespecially preferred as the material forming the ceramic sintered body.

The heat generator 10 is formed by printing a conductive paste on thebase plate 9. The conductive paste includes metal particles, metaloxides, resin, and a solvent. It is preferred that the metal particlesresist oxidation at high temperatures and have a high resistance so thatsufficient heat is produced when a current is passed through the heatgenerator 10. For example, gold, silver, platinum, palladium, copper,tungsten, and nickel may be used. Metal oxides (e.g., lead oxide, zincoxide, silica, boron oxide, alumina, yttria, or titania) may also beused.

As shown in FIG. 4, two pads 10 a, which serve as external connectionterminals, are connected to one end of the heat generator 10. A terminalpin 12 made of a conductive material is soldered to each pad 10 a. Asocket 6 a, which is attached to the distal end of each lead wire 6, isconnected to each terminal pin 12. Current is supplied to the heatgenerator 10 via the lead wires 6 and the terminal pins 12.

Referring to FIG. 4, a plurality of equally spaced threaded holes 13extend through the lip 4. An elastic seal element 14 is arranged on thelip 4 to seal the space between the case 2 and the hot plate 3. Athreaded bore 15 extends through the seal element 14 in correspondencewith each threaded hole 13 of the lip 4. A step 16 extends along theinner surface of the annular seal element 14 to horizontally support thehot plate 3. When the hot plate 3 is supported on the step 16, the topsurface of the seal element 14 is flush with the upper surface of thehot plate 3.

The dimension L between the step 16 and the lower surface of the sealelement 14 is preferably seven millimeters but may be changedarbitrarily within a range of one to ten millimeters. The depth from thetop surface of the seal element 14 to the step 16 is three millimetersto correspond with the thickness 1 of the base plate 9. The depth ischanged in accordance with the thickness 1 of the base plate 9.

An intake port 17 and a discharge port 18 each extend through the bottomwall 2 a of the case 2. Air is sent through the intake port 17 into aspace S1 defined in the case 2. The air is discharged externally fromthe space S1 through the discharge port 18. The intake and dischargeports 17, 18 are separated from each other by a predetermined distance.

The internal surface of the inlet of the intake port 17 is threaded inorder to receive a cooling air supply pipe (not shown), which isconnected to a fluid pump (not shown). The pump sends air, which servesas a cooling fluid, into the space S1 through the supply pipe. Theinternal surface of the outlet of the discharge port 18 is threaded inorder to receive an air discharge pipe (not shown). The cooling air inthe space S1 is discharged through the discharge pipe to a separatelocation.

With reference to FIG. 3, a packing 8 is fitted to the lead wire hole 7.The packing 8 is annular and made of an elastic material such as rubber.The lead wires 6 extend out of the case 2 through the hollow portion ofthe packing 8. The packing 8 seals the space between the lead wires 6and the lead wire hole 7.

The hot plate unit 1 includes a holding frame, or holding ring 21, whichserves as a holder. The holding ring 21 includes a ring body 22 and aplurality of projections 24 (six in the first embodiment). In thisembodiment, the projections 24 are planar tabs. The ring body 22 is madeof spring steel and has a dimension corresponding to that of the lip 4.A plurality of equally spaced screw holes 23 extend through the ringbody 22. The screw holes 23 of the holding ring 21 are formed incorrespondence with the threaded bores 15 of the seal element 14. Theprojections 24 contact the upper surface of the hot plate 3 and projectinward from the ring body 22. As shown by the broken line in FIG. 4,each of the projections 24 is inclined relative to the ring body 22towards the bottom surface of the holding ring 21. The holding ring 21can be produced in a relatively simple manner by, for example, punchinga sheet of spring steel having a thickness of one to three millimetersinto the predetermined shape.

The process for installing the hot plate 3 will now be described. Beforeinstalling the hot plate 3, the terminal pins 12 are soldered to thepads 10 a, and the lead wire sockets 6 a are connected to the terminalpins 12. The lead wires 6 are extended from the case through the leadwire hole 7.

The seal element 14 is then arranged on the lip 4 of the case 2. Theperipheral portion of the hot plate 3 is placed on the step 16. Theholding ring 21 is arranged on the top surface of the seal element 14.The corresponding screw holes 23 of the holding ring 21, the threadedbores 15 of the seal element 14, and the threaded holes 13 are alignedwith one another. The screws 25 are inserted into the aligned holes andbores 13, 15, 23 to fasten the holding ring 21, the seal element 14, andthe lip 4 to one another. As a result, the projections 24 areelastically deformed when they contact the hot plate 3. The hot plate 3is pressed against the step 16 by the projections 24 and clamped betweenthe projections 24 and the seal element 14 (the upper surface of thestep 16). This fixes the hot plate 3 to the case 2 by way of the sealelement 14. The holding ring 21, the hot plate 3, and the case 2 arefixed to one another. Accordingly, the screws 25 serve to fasten theseal element 14 to the case 2 and the holding ring 21 to the case 2.

The method for using the hot plate unit 1 will now be described. Thesilicon wafer W1, to which a photosensitive resin is applied, is placedon the hot plate 3. Current is then supplied to the heat generator 10.As the temperature of the hot plate 3 increases, the silicon wafer W1 isheated. Heating over a predetermined time dries the photosensitiveresin. The flow of current to the heat generator (resistor) 10 is thenstopped.

In this state, the fluid pump is driven to send cooling air into thespace S1 through the intake port 17. The flow of the cooling air isshown schematically by the arrows in FIG. 3. As the cooling air flowstoward the discharge port 18, the air contacts the lower surface of thehot plate 3 in the space S1. Heat is exchanged between the hot plate 3and the cooling air that contacts the hot plate 3. The heated air flowsthrough the discharge port 18 and out of the space S1 to a locationwhere contamination is not a problem. When the hot plate 3 is cooled toa certain temperature, the silicon wafer W1 is removed from the hotplate 3.

The first embodiment has the advantages described below.

(1) The hot plate 3 is cooled by forcing cooling air into the space S1between the case 2 and the hot plate 3. Accordingly, the time requiredfor cooling is decreased in comparison to spontaneous cooling. Thus, thehot plate unit 1 decreases the drying time and improves productivity.Furthermore, leakage of air into the environment of the hot plate unit 1is substantially prevented. Hence, the semiconductor manufacturingapparatus incorporating the hot plate unit 1 and its environment are notcontaminated.

(2) In the first embodiment, the cooling air is efficiently circulatedthrough the intake port 17 and the discharge port 18. That is, theforced cooling air cools the hot plate 3 in the space S1 and returns thetemperature to a low value within a relatively short time.

(3) In the hot plate unit 1, the space between the lip 4 and the lowerperipheral surface of the hot plate 3 is sealed by the seal element 14.Thus, air does not leak between the lip 4 and the hot plate 3. Thisensures that the ambient environment of the hot plate unit 1 isprevented from being contaminated.

(4) In the hot plate unit 1, the substantially sealed space S1 isdefined between the case 2 and the hot plate 3. Projections such as theterminal pins 12 are provided at the lower surface of the hot plate 3.However, these projections are located in the space S1. In other words,the projections are protected by the case 2 and are not exposedexternally from the hot plate unit 1. Accordingly, the bottom surface ofthe case 2 can easily be attached to a support stage (not shown).

(5) In the hot plate unit 1, the ring packing 8 at the lead wire hole 7is arranged in the bottom wall 2 a, and the lead wires 6 are insertedthrough the packing 8. Accordingly, air is prevented from leaking out ofthe apparatus through the lead wire hole 7. This prevents contaminationof the environment of the apparatus.

(6) In the hot plate unit 1 of the first embodiment, the hot plate 3 isattached to the case 2 using the holding ring 21. Thus, screw holes neednot be provided in the periphery of the hot plate 3. That is, when thehot plate 3 is manufactured, holes need not be machined into the hardceramic sintered body. This facilitates production of the hot plate unit1 and decreases the production cost of the hot plate unit 1.

(7) Although the hot plate 3 is fastened by fasteners, such as thescrews 25, the seal element 14 arranged between the hot plate 3 and thecase 2 absorbs the fastening force applied to the hot plate 3. Thus, thefastening force does not damage the hot plate 3.

(8) The hot plate 3 is clamped between the seal element 14 and theholding ring 21. Thus, the hot plate 3 is fixed to the case 2 and isprevented from being loosened.

(9) The holding ring 21 used in the first embodiment includes the ringbody 22, which is made of a spring steel and has a diametersubstantially equal to that of the lip 4, and the projections 24, whichare spaced apart from one another. Accordingly, the projections 24contact the hot plate 3 at different locations. In this state, anacceptable spring force acts on the projections 24 and securely fixesthe hot plate 3 to the lip 4 of the case 2.

(10) The hot plate 3 is fastened by a single holding ring 21. Thus, thenumber of components is decreased in comparison to when using aplurality of holders. Accordingly, the assembly of the hot plate 3 isfacilitated.

Second to seventh embodiments according to the present invention willnow be described. The description centers on parts differing from thefirst embodiment. To avoid redundancy, like or same reference numeralsare given to those components that are the same as the correspondingcomponents of the first embodiment.

As shown in FIG. 5, two intake ports 17 and two discharge ports 18 arerespectively defined in four sleeves fastened to the bottom wall 2 a ofthe case 2. The two intake ports 17 are located at the central portionof the bottom wall 2 a. The discharge ports 18 are located outward fromthe intake ports 17.

When the cooling air is sent into the space S1 through the intake port17 from the fluid pump (not shown), the air is directed verticallyagainst the lower surface of the hot plate 3. The flow of air isschematically shown by the arrows in FIG. 5. The air is heated by theentire hot plate 3 in a substantially uniform manner. The heated airflows out through the discharge ports 18 to a location wherecontamination is no longer a problem.

In addition to the advantages of the first embodiment, the secondembodiment has the advantages described below.

(11) The hot plate unit 1 has two intake ports 17 at the bottom wall 2 aof the case 2. This structure improves the circulation of the coolingair in the space S1. The time required to cool the hot plate 3, with airthat contacts the lower surface of the hot plate 3 is further decreased.Accordingly, the entire hot plate 3 is cooled within a short period oftime.

(12) The intake ports 17 are located at the bottom wall 2 a of the case2. Thus, the air that flows into the case 2 through the intake ports 17is directed vertically against the lower surface of the hot plate 3.This cools the hot plate 3 within a relatively short period of time.

(13) The two intake ports 17 cause the cooling air to contact the entirehot plate 3. Thus, the hot plate 3 is cooled in a uniform manner.

A hot plate unit 1 according to a third embodiment of the presentinvention will now be described with reference to FIG. 6. As shown inFIG. 6, the hot plate unit 1 of the third embodiment does not have thedischarge ports 18 of the second embodiment. A plurality of apertures 19are formed in lieu of the discharge ports 18 in the bottom wall 2 a. Thesimplified structure of the hot plate unit 1 of FIG. 6 saves productioncosts.

A hot plate unit 1 according to a fourth embodiment of the presentinvention will now be described. As shown in FIG. 7(a), the heatgenerator 10 includes three resistors 32-34. Three heating circuits 30are provided to heat the three-part heat generator 10. The three heatingcircuits 30 independently supply the associated resistor 32-34 withpower. In this case, as shown by the diagonal lines in FIG. 7(b), threeheat zones A1-A3 are defined in the hot plate 3.

Referring to FIG. 7(b), a plurality of intake ports 17 (three in thefourth embodiment) are arranged in each heat zone A1-A3, as indicated bythe circles. A plurality of discharge ports 18 (three in the fourthembodiment) are also arranged in each heat zone A1-A3, as indicated bythe triangles. The three intake ports 17 and the three discharge ports18 in each heat zone A1-A3 are equally spaced from one another. Morespecifically, the three intake ports 17 are located at the corners of aright triangle, and the three discharge ports 18 are located at thecorners of another right triangle.

A hot plate unit 1 according to a fifth embodiment of the presentinvention will now be described.

The fifth embodiment differs from the first embodiment in that the sealelement 14 is made of a heat insulating material. Otherwise, the hotplate unit 1 of the fifth embodiment has the same appearance as the hotplate unit 1 of FIGS. 2 to 4.

The seal element 14 may be made of, for example, an inorganic fiber heatinsulating material, such as ceramic fiber or glass wool, a porousresin, such as foam urethane resin or foam styrene resin, orfluororesin. However, it is preferred that the seal element 14 be madeof a ceramic fiber solidified by an inorganic binder since the heatconductivity is low. More specifically, it is preferred that the sealelement 14 be made of a ceramic fiber, such as IBI WOOL (available fromIBIDEN CO., LTD.), that is solidified by silica gel or alumina sol.

When fluororesin is used to form the seal element 14, the seal element14 has superior heat resisting, insulating, and flame-resistingproperties. Fluororesin is used in a temperature range that is widerthan general-purpose synthetic resins and thus is heat resistant.Fluororesin also has a small dielectric loss and is thus a goodinsulator. Furthermore, fluororesin does not burn in air or in a gaswhere the oxygen concentration is 95% or less and thus has goodflame-resistance. The preferred fluororesins are polytetrafluoroethyleneresin (PTFE), tetrafluoroethylene-perfluoroalkylvinylether copolymer(PFA), fluorinated ethylene propylene copolymer resin (FEP), ethylenetetrafluoro ethylene resin (ETFE), polyvinylidene fluoride (PVDF), andethylene chloride trifluoride (PCTFE).

In addition to the advantages of the first embodiment, the fifthembodiment has the advantages described below.

(14) In the fifth embodiment, the hot plate 3 is fastened to the case 2by way of the heat insulative seal element 14. This suppresses thetransfer of heat from the hot plate 3 to the case 2, especially at theperiphery of the hot plate 3. Accordingly, the hot plate unit 1 heatsthe silicon wafer W1 in a uniform manner since high temperatures at theperiphery of the hot plate 3 can be suppressed.

(15) Fluororesin is one of the materials used to form the seal element14. This improves the heat and flame resistance and the insulativecharacteristics of the seal element 14.

A hot plate unit 1 according to a sixth embodiment of the presentinvention will now be described with reference to FIG. 8.

In the hot plate unit 1 of the sixth embodiment, a plurality of holdingplates 31 (six in this embodiment) are used in lieu of the holding ring21 of the fifth embodiment. Each holding plate 31 is arcuate and isformed as if it were cut from the holding ring 21.

Each holding plate 31 is made of spring steel and has a length that is afraction of the circumference of the lip 4 (the circumference of the hotplate 3). In the embodiment of FIG. 8, the circumferential length ofeach holding plate is approximately one twelfth of the circumference ofthe holding ring 21. Screw holes 23 are formed at each end of theholding plates 31. The projections 24 are each formed at one end of eachholding plate 31. The holding plates 31 are equally spaced from oneanother, and the six projections 24, or tabs, press the upper surface ofthe hot plate 3.

Accordingly, in addition to the advantages of the fifth embodiment, thesixth embodiment has the advantages described below.

(16) In the sixth embodiment, the six holding plates 31 are used tofasten the hot plate 3. The spring force of the projections 24 of theholding plates 31 is used to clamp the seal element 14 between the lip 4of the case 2 and the projections 24.

(17) Since the holding plates 31 have a relatively short length,deformation of the plates 31, due to mishandling or other reasons,seldom occurs. Accordingly, the usage of the plurality of holding plates31 to fasten the hot plate 3 maintains the elastic force of theprojections 24 at an appropriate level.

Each of the above embodiments may be modified as described below.

As shown in FIG. 9, a short intake port 17 a and a short discharge port18 a can be provided instead of the intake port 17 and the dischargeport 18 in the first to sixth embodiments. In this case, a fluid flownthrough the intake port 17 a contacts the hot plate 3 at relativelybroader area. Accordingly, the hot plate 3 is cooled more uniformly.

As shown in FIG. 9, the guide sleeves 5 in the first to sixthembodiments may be omitted.

The number of projections 24 in the first to sixth embodiments may bechanged arbitrarily.

The shape and number of the engaging holding plates 31 in the sixthembodiment may be changed arbitrarily.

In the first to sixth embodiments, the holding ring 21 and the holdingplates 31 may be fastened to the seal element 14 or the case 2 usingfasteners other than the screws 25.

In the first to sixth embodiments, the material of the holding ring 21and the holding plates 31 need not have elasticity. Furthermore, theholding ring 21 and the holding plates 31 may be formed integrally withthe seal element 14.

In the first to sixth embodiments, the shape of the projections 24 neednot be plate-like. For example, the projections 24 may be rod-like.

In the first embodiment, the seal element 14 may be eliminated as longas a relatively tight seal is maintained.

In the first to sixth embodiments, the lead wire hole 7 may be formed atsections other than the bottom wall 2 a of the case 2, such as in theside walls of the case 2. Furthermore, the ports 17, 18 may also beprovided at the side walls of the case 2.

In the first, second, and fourth to sixth embodiments, a liquid thatdoes not affect the electric circuits and wiring may be sent into thespace S1 as a cooling fluid as long as the space S1 remains tightlysealed.

In the first to sixth embodiments, a thermocouple for measuringtemperatures may be embedded in the base plate 9. The thermocouple isused to measure the temperature of the hot plate 3 in order to adjustthe values of the voltage or current supplied to the heating circuitaccordingly and control the temperature. In this case, the lead wire ofthe thermocouple also extends from the case 2 through the packing 8.

In the first to sixth embodiments, the number of the intake ports 17 maybe altered. A larger number of intake ports 17 speeds up the cooling ofthe hot plate 3 and cools the hot plate 3 in a more uniform manner.Furthermore, the number of discharge ports 18 may be three or more.

In the fourth embodiment, the intake ports 17 and the discharge ports 18need not be located at the corners of right triangles and may be locatedat arbitrary positions.

In the fourth embodiment, the number of each of the ports 17, 18 may beonly one in the same heat zone.

In the fourth embodiment, the number of heating circuits is not limitedto three. In this case, one or more intake ports 17 need not be providedfor each heating circuit as long as the total number of the intake ports17 is 70 percent of the number of the heating circuits. For example, thenumber of the intake ports 17 may be seven or more if the number of theheating circuits is ten. If the number of the heating circuits is four,the number of the intake ports 17 may be three or more.

In the first to sixth embodiments, the circular hot plate 3 may bechanged to a polygonal one. In this case, the shape of the lip 4, theseal element 14, the holding ring 21 (or the holding plates 31) arechanged in accordance with the shape of the hot plate 3.

It should be apparent to those skilled in the art that the presentinvention may be embodied in many other specific forms without departingfrom the spirit or scope of the invention. Therefore, the presentexamples and embodiments are to be considered as illustrative and notrestrictive, and the invention is not to be limited to the details givenherein, but may be modified within the scope and equivalence of theappended claims.

What is claimed is:
 1. A hot plate unit comprising: a case having a lip;a hot plate having a heat generator, wherein the hot plate is made ofsintered ceramic; and a holder for securing the hot plate to the case,wherein the holder contacts a a peripheral portion of the upper surfaceof the hot plate when the holder is fixed to the case; wherein theholder includes a flat ring, wherein the ring has a diametersubstantially equal to that of the lip; wherein the holder includes aplurality of projections extending inwardly from the ring; wherein theprojections contact separate points on the periphery of the uppersurface of the hot plate.
 2. The hot plate unit according to claim 1,further comprising an annular attachment, wherein the hot plate is fixedto the lip via the annular attachment.
 3. The hot plate unit accordingto claim 2, wherein a step is formed at an inner edge of the attachmentfor supporting the hot plate.
 4. The hot plate unit according to claim1, wherein the holder is plate-like and has at least one inwardlyextending projection, wherein the holder has a hole for receiving afastener.
 5. The hot plate unit according to claim 2, wherein the holderincludes an inwardly projecting potion that is bent toward a lower sideof the holder.
 6. The hot plate unit according to claim 1, wherein theflat ring is made of spring steel.
 7. A hot plate unit comprising: acase having a lip; a hot plate having a heat generator, wherein the hotplate is made of sintered ceramic; and a holder for securing the hotplate to the case, wherein the holder contacts a peripheral portion ofthe upper surface of the hot plate when the holder is fixed to the case;wherein the holder includes a plurality of holding plates, each holdingplate having a length that is a fraction of the length of the lip. 8.The hot plate unit according to claim 7, further comprising an annularattachment, wherein the hot plate is fixed to the lip via the annularattachment.
 9. The hot plate unit according to claim 8, wherein a stepis formed at an inner edge of the attachment for supporting the hotplate.
 10. The hot plate unit according to claim 7, wherein the holderis plate-like and has at least one inwardly extending projection,wherein the holder has a hole for receiving a fastener.
 11. The hotplate unit according to claim 7, wherein the holder includes an inwardlyprojecting portion that is bent toward a lower side of the holder. 12.The hot plate unit according to claim 7, wherein the flat ring is madeof spring steel.